Vehicle fuel systems have for some time incorporated a fuel vapor storage canister with a bed of activated carbon that adsorbs fuel vapors vented from the fuel tank. Stored vapors are later purged by applying engine vacuum to the canister, drawing atmospheric air through the bed to desorb fuel vapor, which is fed to the engine and burned. The fuel tank does not vent just vaporized fuel, however, but a mixture that contains a component of entrained liquid fuel. The efficiency of the carbon bed is reduced by direct contact with liquid fuel, so it is useful to incorporate a liquid trap in the canister to separate the liquid fuel component and keep it isolated from the carbon bed. It is also desirable that the liquid trap be located below the carbon bed, so that trapped fuel cannot slosh out of the trap and into the carbon bed.
Two co-assigned U.S. Pat. Nos., 4,714,485 and 4,750,465, show several embodiments of liquid trap canisters. A potential drawback of the disclosed designs is the fact that the purge tube opens directly into the liquid trap. Therefore, liquid fuel is pulled directly from the trap and fed to the engine to be burned. This may over enrich the engine fuel-air ratio for some vehicles. The purge tube opening can be restricted in size to reduce the rate at which liquid is drawn out, but that also reduces the rate at which vapor may be purged.